Use a PC app named WiiGSC (Wii Game Shortcut Creator)
It will create wad files using the ISO or wbfs game file and it's compatible with all USB loaders
P.S> Make sure you test the wad first using Dolphin on PC or make sure the right anti-brick protection (such as priiloader or bootmii@boot2) because banner bricks can occur with a faulty wad installation
In such case you ll be in trouble